The present invention relates to a method and apparatus for encoding/decoding an image signal with high efficiency.
Conventional image signal encoding/decoding schemes are mainly exemplified by predictive coding and transform coding.
In predictive coding, instead of sending a digitized image signal, a value of a present image signal is predicted by an already coded image signal, and a prediction error signal is sent. Since values of the prediction error signals tend to concentrate near zero, a coded word having a short length is assigned to a quantization level near zero, thereby compressing transmission data. However, in a conventional predictive coding scheme, when a compression ratio of image data is given as 1 bit/pixel, the resolution of a decoded image is rapidly degraded due to quantization noise or the like. It is, therefore, very difficult to compress data at a ratio of 1 bit/pixel or less.
In conversion coding, an image signal is normally divided into blocks each having a proper size, and blocks are expanded in an orthogonal function system, and the expansion coefficients are transmitted. In this case, power tends to concentrate on expansion coefficients corresponding to low-frequency components. In consideration of recognition characteristics, a large number of bits are assigned to a high-power component, while a small number of bits are assigned to a low-power component to achieve coarse quantization, thereby compressing the image signals constituting a one-frame image.
In a conventional transform coding scheme, when the compression ratio is increased, the block structure tends to be apparent, resulting in substantive decrease in the image quality. In order to prevent this, it is preferable to encode a one-frame image without dividing it into blocks. However, the calculation load is then increased to prevent practical implementation. Neither conventional coding scheme can compensate for abrupt changes in image signal levels.